Final answer:
Electrical stimulation of the sinoatrial (SA) node can wake a sleeping animal by initiating a depolarization wave that increases the heart rate.
Step-by-step explanation:
Electrical stimulation of the sinoatrial (SA) node will rouse a sleeping animal from its slumber. The SA node acts as the heart's natural pacemaker, initiating the depolarization wave that spreads across the heart, coordinating its rhythmic contractions. When this node is stimulated, it can result in a sudden acceleration of heart rate, potentially waking an animal from sleep. In addition to the SA node, parasympathetic stimulation such as that originating from the cardioinhibitory center via the vagus nerve can also affect heart rate. However, the vagus nerve typically releases acetylcholine which slows the heart rate by increasing the time before the next spontaneous depolarization occurs.